Mechanical and Civil Engineering Seminar: PhD Thesis Defense

Abstract:

Porosity in solids is ubiquitous throughout engineering applications: inherent in energetic materials, incorporated into shock-absorbing structures, and arising through manufacturing defects such as in additive manufacturing. Understanding the multiscale response of porous materials under dynamic compression is critical for effective design and safety. At the mesoscale, localized shear deformation near pores may lead to structural failure and is believed to drive hot-spot generation and reaction ignition in energetics. However, many details of the mesoscopic response remain unclear. This work presents a novel experimental platform for in-situ investigation of shock-induced pore collapse in solids via plate impact experiments coupled with high-speed quantitative visualization.

The first part of this thesis develops a novel internal digital image correlation (DIC) technique for full-scale dynamic laboratory experiments to measure internal deformations under confinement. The second part applies the internal DIC and shadowgraphy techniques to study the mechanics of pore collapse for individual spherical pores and pore arrays. First-of-its-kind internal strain measurements reveal concentrations around the collapsing pores and shear bands emanating from the pore surface. Raw images reveal the propagation of shear cracks at higher impact stresses. Moreover, pore-pore interactions shift the failure mode thresholds to lower impact stresses. Finally, theoretical models and thermo-viscoplastic numerical simulations are employed to elucidate the mechanisms governing the observed responses.

These novel in-situ observations of adiabatic shear banding and improved characterization of crack propagation during pore collapse advance understanding of the mesoscale response and failure of porous solids under shock compression.